Even a moonless night sky with an absence of artificial light is also not wholly dark. Its brightness depends on (1) zodiacal light, (2) faint unresolved stars and diffuse galactic light due to atomic processes within our galaxy, (3) diffuse extra-galactic light, and (4) air-glow and aurora [1]}, [2]}. Out of the above-discussed four sources, the first three sources are independent of the observational site, whereas the fourth one depends on the site and time of observation [3]}, [4]}, [1]}. A good astronomical site is characterized by having a low night sky brightness. Sky brightness can be estimated using the formula given by [3]} (see also [7]}):
\(S = -2.5\ log\frac{C_{sky}}{C_*} + 2.5\ log\frac{E_{sky}}{E_*} + \kappa _\lambda X_* + M_*,\) 
